High Efficiency and Optical Anisotropy in Double-Heterojunction Nanorod Light-Emitting Diodes

Abstract

Recent advances in colloidal quantum dot light-emitting diodes (QD-LEDs) have led to efficiencies and brightness that rival the best organic LEDs. Nearly ideal internal quantum efficiency being achieved leaves light outcoupling as the only remaining means to improve external quantum efficiency (EQE) but that might require radically different device design and reoptimization. However, the current state-of-the-art QD-LEDs are based on spherical core/shell QDs, and the effects of shape and optical anisotropy remain essentially unexplored. Here, we demonstrate solution-processed, red-emitting double-heterojunction nanorod (DHNR)-LEDs with efficient hole transport exhibiting low threshold voltage and high brightness (76,000 cd m(-2)) and efficiencies (EQE = 12%, current efficiency = 27.5 cd A(-1), and power efficiency = 34.6 lm W(-1)). EQE exceeding the expected upper limit of ∼ 8% (based on ∼ 20% light outcoupling and solution photoluminescence quantum yield of ∼ 40%) suggests shape anisotropy and directional band offsets designed into DHNRs play an important role in enhancing light outcoupling.

High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers

“…These elongated heterostructures combine the advantages of high emission yield 49,50 , suppressed Auger recombination 51,52 and stability 53 of spherical core/shell heterostructures with the ability of nanorods to assemble into ordered anisotropic solids, which has recently been exploited to realize polarized QDLEDs 40,54 . In agreement with previous observations 40,41 , the scanning electron micrograph of a representative DiR film on ITO/PEDOT:PSS/PVK, before further deposition of the ETL and the metal cathode, shows a mosaic of ordered domains of DiRs aligned along their c-axis and further highlights the excellent layer homogeneity achievable by spin coating deposition (Fig.1c).…”

“…To test this effect, we fabricated and tested QD-LEDs embedding spherical CdSe/CdS core/shell QDs (core diameter= 4 nm, shell thickness=2 nm, with Φ PL~8 5%) and the different polymer ETLs. The data, reported in Supporting Fig.S5, confirm the superior performance of anisotropic structures, revealing approximately ten-times lower EQEs with respect to the DiR-based LEDs, in agreement with recent observations 41 .…”

“…Nevertheless, CdSe/CdS/ZnSe nanorods have been recently shown to boost both the efficiency and brightness of QD-LEDs with respect to spherical QDs, possibly due to favorable band offset and enhanced light outcoupling 41 , thus making elongated structures promising (diameter = 5.1 nm) overcoated with a 6.8 nm thick and 26.8 nm long CdS rod (see schematic in the top panel in Fig.1d and the transmission electron micrograph in Supporting Fig.S1). These elongated heterostructures combine the advantages of high emission yield 49,50 , suppressed Auger recombination 51,52 and stability 53 of spherical core/shell heterostructures with the ability of nanorods to assemble into ordered anisotropic solids, which has recently been exploited to realize polarized QDLEDs 40,54 .…”

“…Recently, variable angle spectroscopic ellipsometry measurements by Nam et al 41 indicated optical anisotropy in heterostructured nanorods films resulting from the emissive dipoles being mostly parallel to the device surface and leading to suppression of optical losses due to directional emission towards the device edges [75][76][77] . However, the angular profile of the EL intensity for DiR-and QD-based LEDs reported in Supporting Fig.S6 shows that both devices exhibit Lambertian emission distribution, which might suggest that such effect is not dominant in our DiR-LEDs possibly due to random orientation of the DiRs in the 6-7 monolayers thick active layer.…”

Flexible quantum dot light-emitting diodes for next-generation displays

“…2k, l). [53][54][55] In this structure, two CdSe emitters are directly connected to CdS nanorods and the remaining surface of CdSe is passivated by ZnSe. Remarkably, the obtained peak EQE (12%) was higher than the expected upper limit (8%) considering their PLQY (40%).…”